1. Field of the Invention
The present invention relates to an optical repeater device (ORD) and an optical transmission system. More specifically, it relates to the ORD, and to an optical transmission system of a passive double star (PDS) type comprising a repeater of the ORD.
2. Description of Related Art
Regarding an optical network, such as a fiber to the home (FTTH), a cable television (CATV), and the like, an optical transmission system of a passive double star (PDS) type is used, in which a passive device is provided for a branching part branching a cable into the number of users on the way of a path from a central office to the users. Regarding the passive device, there is provided such as a splitter for example.
Such the splitter is to be connected with an optical fiber to be taken out for the user side from each optical subscriber unit (OSU) provided in an optical line terminal (OLT) at the central office. Moreover, optical paths branched by the splitter are to be connected with optical network units (ONU) for a plurality of users, via a plurality of optical fibers.
According to such the optical transmission system of the PDS type in which the OLT and a plurality of ONUs are connected using the optical fibers and the splitter, data are to be transmitted using an optical signal of only one wavelength. Hence, it is had for improving a large capacity.
On the contrary, a system for being able to perform a wavelength division multiplexing (WDM) communication, with using the ONU in such the existing optical transmission system without changing any, is reported in the below described patent document 1.
Regarding the system, as shown in FIG. 12, for optical fibers 104 connecting between an OSU 102 in an OLT 101 and a splitter 103, a first multiplexer/de-multiplexer (MUX/DEMUX) 105, an optical transmission path 106, a second MUX/DEMUX 107, and then a wavelength converter 108 are to be connected in order therewith, as a configuration thereof. Thus, according to the configuration, the existing splitter 103 and the existing ONU 109 are become able to be used. Moreover, between the OLT 101 and the ONU 109, a sufficient allowable dissipation becomes to be given, and then a distance restriction from dissipation becomes able to be removed. Therefore, it is reported therein that the large capacity and a long distance communication becomes able to be improved.
Regarding the wavelength converter 108, as shown in FIG. 13, a common configuration for wavelength conversion is to be adopted.
The common wavelength converter comprises two of 3 (dB) couplers 110a and 110b to be connected with the second MUX/DEMUX 107 and the splitter 103 respectively. Moreover, for both up and down as two pathways signalling system between the two of 3 (dB) couplers 110a and 110b, it comprises opto-electric conversion devices 111 and 121 as light receiving elements, 2R/3R receiver circuits 112 and 122, driver circuits 113 and 123 for the light receiving elements, light emission elements 114 and 124, and isolators 115 and 125, to be connected in order toward the signal propagation direction respectively. Thus, there is provided a configuration thereby.
Moreover, such as disclosed in the patent document 1, there is reported a configuration regarding the wavelength converter 108, with omitting the light receiving element 121, the receiver circuit 122, the driver circuit 123, and the light emission element 124 on the down signaling pathway.
Furthermore, such as in a below disclosed nonpatent document 1, there is reported a configuration that, for a down signaling system of the optical signaling pathways between the OLT and the splitter, a 2.488 Gbit/s converter, a dense wavelength division multiplexer (DWDM), and another converter are connected in order toward the signal propagation direction therein. Moreover, for a up signaling system therein, a converter, a DWDM, and an optical channel filter are connected in order toward the signal traveling direction, as the configuration. And then the long distance between the OLT and the ONU becomes to be improved. Moreover, the DWDM is to be comprised of a semiconductor laser, a modulator, an optical receiver, a variable optical attenuator (VOA) and the MUX/DEMUX.    [Patent Document 1] Japanese Patent Application Publication No. 2002-261697    [Nonpatent Document 1] R. P. Davey et al. “DWDM reach extension of a GPON to 135 km” PDP35, 2005, Optical Society of America
Regarding the optical transmission system according to the patent document 1 and the nonpatent document 1, there are some problems as described below.
A first problem is that a load of time and cost for exchanging an existing OLT is quite large, due to realizing impossible with using the existing OLT regarding the optical transmission system according to the patent document 1 and the nonpatent document 1. The reason is described as below.
Regarding the existing OLT, there are some for example regulations compliant, such as a gigabit passive optical network (G-PON) to be pursuant to the international telecommunication union telecommunication standardization sector (ITU-T) standard G.984.x, a gigabit Ethernet (the registered trade name) PON (GE-PON) to be pursuant to the IEEE 802.3ah standard of the Ethernet in the first mile (EFM), and the like. Meanwhile, regarding the OLT regulated by the G-PON and the GE-PON, a sending wavelength range is determined between 1.48 and 1.50 μm, and the receiving wavelength is determined as a range between 1.26 and 1.36 μm. However, essentially such the existing OLT is not suitable for exclusive use of WDM transmission.
Regarding the system according to the patent document 1, signals of different wavelengths are required to be generated in the OLT itself, and to be transmitted to the MUX/DEMUX. Therefore, the existing OLT becomes necessary to be changed for WDM transmission.
Moreover, for an optical amplifying system regarding the DWDM according to the nonpatent document 1, because of a structure for receiving a signal of 1.55 μm wavelength-band therein, the existing OLT of the receiving wavelength between 1.26 and 1.36 μm is not able to be used for direct receiving.
A second problem is that, regarding the PDS type system of time division multiplex (TDM), such as the G-PON, the GE-PON, or the like, from a plurality of ONUs connected with single splitter, signals are not able to be transmitted to the OLT at the same time. Hence, upward signals from the ONU side to the OLT become to be burst signals. And then a preamble signal included in the burst signals cannot help but be disappeared.
In the case of relaying the burst signal by the wavelength converter, shown as signals surrounded by dashed lines in FIG. 13, a burst signal S1 is to be input from the ONU 109 to the wavelength converter 108, via the splitter 103. And then at the period of the burst signal S1 being processed in the receiver circuit 112, a part of the preamble signal for signal identification included in the burst signal S1 cannot help but be disappeared. Moreover, the disappearance of the preamble signal is occurred for the following reason.
That is to say, regarding the upward signal, a level of a signal input from the ONU 109 to the wavelength converter 108 is different from each of the ONU 109. And then in the case of receiving a burst signal from each ONU 109, in the receiver circuit 112, for identifying a signal level of “0” and “1”, it needs to be processed as feedforward based on the received signal. Hence, a certain amount of time is required for such the process. Normally, as the burst signal, a preamble signal is added onto a data signal therein. And then during the certain amount of time for identifying the signal level in the wavelength converter 108, the corresponding amount of the preamble signals cannot help but be disappeared.
In other words, for a long distance transmission regarding the optical transmission system, to avoid a waveform degradation, at the receiver circuit 112 in the wavelength converter 108, a 3R process, that is to say, an equalized amplifying as a re-shaping, an identification and a regeneration, and a re-timing become necessary to be performed. However, for the re-timing process, a data and a clock are regenerated based on the received signal. Hence, a certain amount of time is required for the process, and then the disappearance of the preamble signal is occurred. Meanwhile, for regenerating the stable data and the stable clock based on the burst signal, generally, with a time constant longer, the receiving signals are better to be taken in the longer period of time thereinto. And then it becomes a trade off relation with the disappearance of the preamble signal. Therefore, in the case of taking time for regenerating the data and the clock too short, the data and the clock cannot help but be regenerated properly.
As above mentioned, in the case of using the wavelength converter as a repeater, with such the ordinary methods, at least a part of the preamble signal cannot help but be disappeared. And then comparing to a case without using the repeater, the preamble signal to be input into the OLT cannot help but be shorter. Hence, at the OLT side, there is a case occurred that a burst signal becomes unable to be received.
On the contrary, by presetting bit numbers larger regarding the preamble signal included in the burst signal, the disappearance of the preamble signal at the repeater is to be supplemented, as a method to be adopted and may be considered. However, a bit number ratio of the preamble signal occupies in the burst signal becomes higher. And then the bit number ratio of the data signal occupies in the burst signal cannot help but be lower. Hence, a transmission capacity (throughput) as a total system cannot help but be decreased.
A third problem is that, according to the patent document 1 and the nonpatent document 1, the upward burst signal from the ONU to the OLT is to be directly relayed, as the system. Therefore, all devices and components to be used in the upward section of the system are necessary to be corresponding to the burst signal. Hence, a component for continuous signals as more simple configuration and lower cost is not able to be used therefor. And then the cost for the total system cannot help but be increased.